The electrochemical couple of a lithium metal anode with a pyrite or iron disulfide cathode has long been recognized as a theoretically high-energy couple. Hereinafter, “pyrite” and “iron disulfide” will be used interchangeably. Lithium metal possesses the lowest density of any metal and provides a volumetric energy density of 2062 mAh/cubic centimeter and a gravimetric energy density of 3861.7 mAh/gram. Pyrite offers advantageous energy opportunities as a result of its ability to undergo a four electron reduction, and has a volumetric energy density of 4307 mAh/cubic centimeter and a gravimetric energy density of 893.58 mAh/gram.
There are however many challenges in achieving a commercially viable cell with this particular electrochemical couple. One key challenge is how to use internal cell volume efficiently. It is known that this electrochemical system results in a volume increase upon discharge and the accompanying formation of reaction products. It is therefore necessary that the cell design incorporate sufficient void volume to accommodate this volume increase. It will be appreciated then, that as the discharge efficiency of the cell increases, additional reaction products will be generated causing incremental volume increases that must be accommodated by the incorporation of sufficient void volume within the cell.
Attempts to improve the energy density of the cell by increasing the density of the cathode present additional challenges. First, it will be appreciated that an increase in the density of the cathode will result in less void volume within this electrode to accommodate the reaction products, in turn requiring that alternative void sites within the cell be provided. Further, the densification of the cathode through an increase in the calendering force applied to the coated electrode stock can result in a stretching of the metallic foil substrate mat functions as the cathode current collector. Such stretching can compromise the uniformity of the coating layer and can lead to wrinkling, cracking and ultimately the separation of all or portions of the coating layer from the substrate.
In the interest of accommodating the increase in volume relating to the reaction products for the lithium/iron disulfide electrochemical couple while also improving the cell discharge efficiency and cell capacity, it will therefore be appreciated that the volume occupied by non-reactive internal cell components should be minimized to the extent possible. In this regard, use of lithium metal foil as the anode obviates the need for a discrete anode current collector, since the lithium foil is sufficiently conductive. However, lithium foil has a relatively low tensile strength and as a result can undergo stretching and thinning causing localized regions of reduced anode capacity. In a pronounced case, the thinning can be aggravated to the point of disconnects within the lithium anode. Various solutions to the problem of lithium foil weakness have been proposed, including, the design of cells with thicker lithium foils, separate anode current collectors, or lithium anodes with regions of reduced or non-ionic transport. These solutions typically result in an anode overbalance in the cell and are not efficient or volumetrically satisfactory. The use of excess lithium in the cell is also costly since metallic lithium foil is a relatively costly material.
There is therefore a need for a nonaqueous lithium/iron disulfide cell with an increased energy density and discharge efficiency that accommodates the volume increase of the reaction products generated during discharge. There is further a need for such a nonaqueous cell having a dense cathode with good adhesion to the current collector substrate without sacrificing the uniformity of the cathode coating layer. There is further a need for such a nonaqueous cell that reduces the anode to cathode cell balance without sacrificing the integrity of the anode.